Drilling fluid and method for drilling in coal-containing formations

1. A method for drilling in a coal containing formation, the method comprising:

providing a mixed metal-viscosified drilling fluid including clay, a mixed metal-viscosifier and at least 0.05% (w/v) calcium sulfate;

circulating the drilling fluid through the well; and

drilling into the coal containing formation,

provided that the calcium sulfate is added to protect an electrostatic interaction between the clay and the mixed metal-viscosifier, thereby maintaining the viscosity of the drilling fluid at a yield point of greater than 10 Pa.

2. The method of claim 1 wherein providing the mixed metal-viscosified drilling fluid includes obtaining a drilling fluid with a yield point greater than 10 Pa.

3. The method of claim 1 further comprising continuing circulation with entrained coal fines in the drilling fluid.

4. The method of claim 1 wherein circulating the drilling fluid is initiated prior to drilling into the coal containing formation.

5. The method of claim 1 wherein circulating the drilling fluid is maintained while a coal seam is open to the drilling fluid.

6. The method of claim 1 wherein circulating the drilling fluid is initiated when the drilling process is initiated.

7. The method of claim 1 wherein the calcium sulfate is in the form of gypsum.

8. The method of claim 1 wherein the drilling fluid further comprises an amount of potassium salt selected from the group consisting of potassium sulfate, potassium chloride, potassium acetate and potassium formate.

9. The method of claim 1 wherein the clay is bentonite.

10. The method of claim 9 wherein providing the mixed metal-viscosified drilling fluid includes mixing an aqueous-based drilling fluid including 15 to 50 kg/m3 bentonite, a mixed metal viscosifier at a weight ratio of 1:8 to 1:12 viscosifier to bentonite, a base to maintain a pH above about 10.0 and the at least 0.05% (w/v) calcium sulfate.

11. The method of claim 9 wherein providing the mixed metal-viscosified drilling fluid includes mixing an aqueous-based drilling fluid including about 25 to 45 kg/m3 bentonite, a mixed metal viscosifier in a quantity of about 1:10 mixed metal viscosifier to bentonite with a pH controlled to greater than pH 11 and 0.05 to 1.0% calcium sulfate.

12. The method of claim 9 wherein providing the mixed metal-viscosified drilling fluid includes:

mixing bentonite in water to form a bentonite mixture;

adding a mixed metal viscosifier to the bentonite mixture;

adjusting pH to greater than about pH 10; and

adding the calcium sulfate.

13. The method of claim 12 further comprising adding any of fluid loss control additives and/or lost circulation materials.

14. The method of claim 13 wherein the pH is adjusted using caustic soda or caustic potash.

15. The method of claim 9 wherein the bentonite is in the form of untreated bentonite.

16. The method of claim 1 wherein adding the calcium sulfate is at a weight ratio of 1:2.5 to 1:90 calcium sulfate to clay.

17. A method for drilling a wellbore, the method comprising:

providing a mixed metal-viscosified drilling fluid including clay and a mixed metal-viscosifier;

monitoring rheology of the mixed metal-viscosified drilling fluid; and,

adding calcium sulfate in an amount of at least 0.05% (w/v) to the mixed metal-viscosified drilling fluid to avoid adverse rheological changes, provided that the calcium sulfate is added to protect an electrostatic interaction between the clay and the mixed metal-viscosifier, thereby maintaining the viscosity of the drilling fluid at a yield point of greater than 10 Pa.

18. The method of claim 17 wherein the clay is bentonite.

19. The method of claim 18 wherein providing the mixed metal-viscosified drilling fluid includes mixing an aqueous-based drilling fluid including 15 to 50 kg/m3 bentonite, the mixed metal viscosifier at a weight ratio of 1:8 to 1:12 viscosifier to bentonite and a base to maintain a pH above about 10.0.

20. The method of claim 18 wherein providing the mixed metal-viscosified drilling fluid includes mixing the aqueous-based drilling fluid including about 25 to 45 kg/m3 bentonite and the mixed metal viscosifier in a quantity of about 1:10 of mixed metal viscosifier to bentonite with a pH controlled to greater than pH 11.

21. The method of claim 18 wherein providing the mixed metal-viscosified drilling fluid includes:

mixing bentonite in water to form a bentonite mixture;

adding the mixed metal viscosifier to the bentonite mixture; and

adjusting pH to greater than about pH 10.

22. The method of claim 21 further comprising adding any of fluid loss control additives and/or lost circulation materials.

23. The method of claim 17 wherein adding includes adding 0.05 to 1.0% calcium sulfate.

24. The method of claim 17 wherein adverse rheological changes includes viscosity thinning.

25. The method of claim 17 the calcium sulfate is added prior to drilling into a coal seam.

26. The method of claim 17 the calcium sulfate is added prior to initiating the drilling process.

27. The method of claim 17 wherein the calcium sulfate is in the form of gypsum.

28. The method of claim 17 wherein adding the calcium sulfate is at a weight ratio of 1:2.5 to 1:90 calcium sulfate to clay.

29. A method for drilling a wellbore, the method comprising:

providing a drilling fluid, the drilling fluid being aqueous and clay-based including 15 to 50 kg/m 3 clay and a mixed metal viscosifier at a weight ratio of 1:8 to 1:12 viscosifier to clay;

monitoring rheology of the drilling fluid; and

adding to the drilling fluid calcium sulfate at a weight ratio of 1:2.5 to 1:90 calcium sulfate to clay to avoid adverse rheological changes,

provided that the calcium sulfate is added to protect an electrostatic interaction between the clay and the mixed metal-viscosifier, thereby maintaining the viscosity of the drilling fluid at a yield point of greater than 10 Pa.

30. The method of claim 29 wherein providing the drilling fluid includes adding a base to maintain a pH above about 10.0.

31. The method of claim 29 wherein the clay is bentonite and providing the drilling fluid includes:

mixing bentonite in water to form a bentonite mixture;

adding the mixed metal viscosifier to the bentonite mixture; and

adjusting pH to greater than about pH 10.

32. The method of claim 31 further comprising adding any of fluid loss control additives and/or lost circulation materials.

33. The method of claim 29 wherein adding the calcium sulfate is conducted prior to drilling into a coal seam.

34. The method of claim 29 wherein adding the calcium sulfate is conducted prior to initiating the drilling process.

35. The method of claim 29 wherein the calcium sulfate is in the form of gypsum.